Magnetic resonance imaging (MRI) is a state of the art imaging technology which allows cross sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both NMR and MRI is the fact, that atomic nuclei with non-zero spin have a magnetic moment. In medical imaging, usually nuclei of hydrogen atoms are studied since they are present in the body in high concentrations like for example water. The nuclear spin of elementary particles can resonate at a resonance frequency, if a strong DC magnetic field is applied. This magnet resonance (MR) frequency is determined by the level of the magnetic flux. In an MRI scanner, the magnetic field matches a selected resonance frequency only at one position in space. Only at this position the presence of these particles can be detected. By varying this position step-by-step, an image can be measured. In practice, more sophisticated algorithms are used to achieve the image in a reasonable time from e.g. ‘slices’ of the investigated volume. Typical resonance frequencies are in the range from 40 MHz to 120 MHz, corresponding to magnetic flux levels in the range of 1 T to 3 T.
The needed strong DC magnet field (B0 field) is typically generated by superconducting magnets. In order to vary this field, such that it matches a given radio frequency only at one position, a field gradient is generated using gradient coils. The field gradient can vary over time to achieve a scan. The frequency range in the gradient coils is low, and reaches up to a maximum of 10 kHz.
To excite nuclear resonances, the RF coil generates a high frequency magnetic field at the nuclear resonance. The magnetic field must direct in radial direction with respect to the axis of the MRI scanner. To achieve a radial magnetic field in all directions, a rotating field is used, which points in any radial direction at one point of time during one period. This is achieved by using a so called ‘birdcage’ arrangement. Currents in opposing slabs of the bird cage flow in opposing direction and thus generate a radial field. Currents in neighbored slabs have a phase shift, such that a field rotates.
To measure nuclear resonances, “sensor” or “receiver” coils are placed close to the region of interest, e.g. on the patient. These coils must be oriented such that their axis points approximately in radial direction with respect to the axis of the MRI scanner. Often, a number of sensor coils are connected to a complete module, e.g. such a module may consist of 4×4 individual sensor coils. The module also includes additional electronics to process the measured signals.
The sensor coil modules are typically connected to the MRI system by cables. However, the usage of cables to connect the coil modules to the MRI system has various disadvantages. For example, the comparably stiff cables may be the cause for an unwanted displacement of the modules during movement of the patient. Furthermore, in some cases common mode currents may be induced in the cables which deteriorate the image quality and even may cause harm to the patient. Therefore, it is advantageous to omit the cables.
For example, WO 2006/06768282 discloses a device for performing magnetic resonance imaging of a body, wherein power is provided to sensor coils wirelessly. The power is transmitted by magnetic induction. Transmitter coils, which are integrated in the MRI tube, generate an alternating magnetic field. The power receiver coils are integrated in the sensor coil module. The alternating magnetic field induces a voltage in the power receiver coil, which is used to power the module.
However, there may arise the problem that unwanted disturbances of the sensor coils may occur since the sensor coils are extremely sensitive against any disturbances in the RF (radio frequency) range around the MR frequency.
Therefore there is a need for an improved nuclear magnetic resonance imaging apparatus, an improved nuclear magnetic resonance imaging receiving coil, an improved method of performing nuclear magnetic resonance imaging, an improved method of acquiring magnetic resonance imaging signals in a receiver coil and an improved computer program product, which overcomes the before mentioned problems.